1. Field of the Invention.
The invention relates to a switched mode mains power supply unit having a transformer, which comprises a primary winding and at least one secondary winding as well as an auxiliary winding, having a switch controlled by a control circuit, by means of which switch the primary winding is connected to an input direct voltage, having, connected downstream of the secondary winding, a rectifier together with a filtering means for producing an output direct voltage and having a regulator means for producing a first control variable for the control circuit in dependence upon the deviation of the output voltage from a first set value, wherein the pulse duty ratio and/or the frequency of the control signal of the controlled switch depends upon the control variable, and also having an auxiliary rectifier connected downstream of the auxiliary winding in order to produce an auxiliary direct voltage for the control circuit and/or other consumers.
Switched mode mains power supply units of this type are used extensively for industrial applications, wherein usually from the alternating current mains by rectification a so-called intermediate circuit voltage is obtained of an order of magnitude of 325 Volts to a maximum of 400 Volts. This intermediate circuit voltage is then the input voltage of the actual switched mode mains power supply unit.
In practice the requirement often arises for parallel connection of a plurality of switched mode mains power supply units, which are in principle of the same type, either because it is desired to cover the power requirement with the aid of a plurality of small units or because, owing to a higher power requirement arising later, it is desired to refit an existing power supply by connecting in one or a plurality of further mains power supply units.
There are different possible approaches for ensuring a parallel connection of a plurality of mains power supply units:
The operating condition of the individual parallel-connected mains power supply units can be detected and controlled centrally, for example, in a microcontroller, computer, PC or the like, or a master-slave structure can be constructed in which one unit assumes a prominent position.
The units which are connected in parallel have no connection to a higher element but are, however of equal status and of the same construction and have their own data channel for data exchange or the mains power supply units are carefully balanced with each other during installation of the parallel connection.
The equal-status, structurally-alike units have no individual connection for the exchange of data and are also not balanced in order to render it possible for them to be connected in parallel. Units of this type should be able to be used with no alteration for individual operation or for parallel connection.
The invention relates to the last-named case and relates to the specific problem that owing to component and manufacturing tolerances the individual mains power supply units can or will have different open-circuit output voltages. If two mains power supply units are connected in parallel the operating condition arises in open-circuit and partial load such that the unit with the higher output voltage during open-circuit determines the common output voltage and during partial load also takes over the whole charge current, and the regulator of the second mains power supply unit establishes an excessively high actual value for the output voltage. The regulator of the unit with a smaller open-circuit output voltage accordingly attempts to lower the output voltage and it reduces the pulse duty ratio and/or frequency of the controlled switch, wherein--owing to the parallel connection of the mains power supply units--it is not the output voltage which can fall but the auxiliary direct voltage, which feeds the control circuit and, where applicable, other components, is lowered until it achieves a value which is inadmissibly low for the current supply to the control circuit and possibly other circuits. This can lead to the complete switch-off of the switching and control electronics and therefore to a "shut-down" of the mains power supply unit. If the current supply is in this condition, i.e., the auxiliary direct voltage is so low in a switched mode mains power supply unit that the control circuit and other electronic/logic parts are switched off and if then the switched mode mains power supply unit switched off in this way must take over current, for example, because of a change in charge, it is first necessary to build up the auxiliary direct voltage so that the control circuit and the controlled switch can be returned to operation. This process requires a certain amount of time of which the duration is determined, for example, by the size of a capacitor on the auxiliary direct voltage and by a relatively high-ohmic auxiliary resistor charging this capacitor from an intermediate circuit voltage. During this time the output voltage in the case of corresponding charge ratios can be interrupted if a great deal of expense has not been invested in buffer capacitors or the like, wherein such interruptions in critical applications are extremely undesirable since they can lead, for example, to loss of data. The same is true in essence even in parallel circuits of more than two mains power supply units.
A switched mode mains power supply unit is known from EP 0 585 789 A1 in which an auxiliary direct voltage produced by an auxiliary winding and an auxiliary rectifier is supplied via the collector-emitter-path of the transistor of an optical coupling device to the control circuit. The voltage arriving therein accordingly changes with the output direct voltage and is used for regulation. The integrated circuit used for the control circuit is thus dimensioned in such a way that the voltage varying in this way also serves to supply current. Regulation of this voltage is generally not considered and the afore-mentioned problems in the parallel connection of a plurality of mains power supply units would consequently occur.
2. Description of the Prior Art.
EP 0 509 343 A2 discloses a feed switched mode mains power supply unit in which an auxiliary direct voltage is used for the control circuit, of which the origin is not explained in detail. The transformer of the mains power supply unit has three windings, namely a primary winding, a secondary winding and a demagnetising winding with a return diode.
The flyback converter shown in DE 35 08 895 A1 has a regulation for two separate output voltages, to which two secondary windings of the transformer are allocated. An auxiliary direct voltage is in this case drawn from the input direct voltage via a longitudinal regulator not explained in more detail, wherein this regulation is completely independent of the actual regulation in the switched mode mains power supply unit. Switching concepts of this type are not effective in particular if the input voltages are relatively high, for example, 230.multidot..sqroot.2 Volt. The auxiliary direct voltage is in particular only of the order of magnitude of 10 Volts so that the achievement of an auxiliary direct voltage without an auxiliary winding of the transformer consumes energy. The circuit disclosed in the document accordingly also comprises no selector circuit relating to the regulation of the auxiliary direct voltage and/or the regulation of the output voltage.
One object of the invention is thus, in a switched mode mains power supply unit of the representative type, i.e. in one in which the auxiliary direct voltage is produced for the control circuit and possibly other logic circuits with an auxiliary rectifier by means of an auxiliary winding of the transformer, to provide the possibility of preventing an undesired lowering of the auxiliary direct voltage and the associated shut-down of the control circuit and/or further circuit parts in the case of parallel connection of a plurality of switched mode mains power supply units.
This object is achieved with a switched mode mains power supply unit of the classified type, which is characterised in accordance with the invention by an auxiliary regulator to produce a second control variable for the control circuit in dependence upon the deviation of the auxiliary direct voltage from a second set value and by a selector circuit for priority supply of the control variable of that regulator to the control circuit, the actual value of which lies below the associated set value.
If, in the case of switched mode mains power supply units connected in parallel, the above-described operating condition occurs in which a regulator with lower output open-circuit voltage as a result of parallel connections might attempt to lower the output voltage this can only take place to a point at which the auxiliary direct voltage falls to a tenable minimum value. Then the regulation is undertaken by the auxiliary regulator which then ensures the maintenance of the auxiliary direct voltage by means of the control circuit. This condition only changes when the common output voltage falls below its set value because of charging whereby the output voltage regulator comes into action again and the auxiliary regulator moves into the background.
Further features of the invention are characterised in the dependent claims.